Give ModuleFiles an index, so that we can use indexed vectors rather
than DenseMaps and SmallPtrSets for module-visitation data. ~2.6%
speedup for modules.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@173081 91177308-0d34-0410-b5e6-96231b3b80d8